The present invention relates to a water and fat separation image forming method, a magnetic resonance imaging apparatus, a reference peak phase detecting method and a reference peak position detecting method. More in details, the present invention relates to a water and fat separation image forming method and a magnetic resonance imaging apparatus capable of separately and preferably forming a water image and a fat image even in the case in which a plurality of independent signal regions are present in a complex number image taken by utilizing magnetic resonance. Further, the present invention relates to a reference peak phase detecting method and a reference peak position detecting method capable of simply detecting a phase or a position of a reference peak in a distribution satisfying a specific condition.
There is disclosed a water and fat separation image forming method for taking a complex number image by utilizing magnetic resonance to provide a phase difference between water and fat and separately forming a water image and a fat image from the complex number image by utilizing the phase difference between water and fat in, for example, xe2x80x9cSMRM85vol. 1 pp. 172-173: Zvi Paltiel, Amir Ban (Elscient MRI Center)xe2x80x9d.
FIG. 1 shows a flow of a conventional water and fat separation image forming method.
At step J1, an image is taken by utilizing magnetic resonance to provide a phase difference between water and fat and form a complex number image.
At step J3xe2x80x2, a phase error caused by a non-uniformity of a magnetic field is corrected.
At step J5xe2x80x2, a water image and fat image are separately formed by utilizing the phase difference between water and fat from the complex number image the phase of which has been corrected.
According to the conventional water and fat separation image forming method, as shown in FIG. 2, when there is only one signal region N in the complex number image (that is, when the signal region S is not divided by a noise region N), the water image and fat image can properly be formed to separate from each other.
However, when there are a plurality of signal regions S1 through S3 in the complex number image as shown by FIG. 3, (that is, the signal regions S1 through S3 are divided by a noise region N2), there poses a problem in which the water image and the fat image cannot properly be formed to separate from each other.
It is a first object of the present invention to provide a water and fat separation image forming method and a magnetic resonance imaging apparatus capable of separately and preferably forming a water image and a fat image even in the case in which there are present a plurality of independent signal regions in a complex number image taken by utilizing magnetic resonance.
Further, it is a second object of the present invention to provide a reference peak phase detecting method which is a method capable of being utilized in implementing the water and fat separation image forming method and the magnetic resonance imaging apparatus according to the first object and which can simply detect a phase of a reference peak in a distribution satisfying a specific condition.
Further, it is a third object of the present invention to provide a reference peak position detecting method which is a method generalizing the reference peak phase detecting method according to the second object and which can simply detect a position of a reference peak in a distribution satisfying a specific condition.
According to a first aspect of the present invention, there is provided a water and fat separation image forming method characterized in a water and fat separation image forming method for taking a complex number image by utilizing magnetic resonance to provide a phase difference between water and fat and separately forming a water image and a fat image from the complex number image by utilizing the phase difference between water and fat, wherein when there are present a plurality of independent signal regions in the complex number image, phases are respectively corrected in the respective signal regions, phase shifts among the respective signal regions are corrected, and then, the water image and the fat image are separately formed from the complex number image by utilizing the phase difference between water and fat.
When there are a plurality of signal regions in a complex number image, the reason that a water image and a fat image cannot properly be formed to separate is that phase shifts are present among the plurality of signal regions. That is, in one signal region, phase error caused by a non-uniformity in a magnetic field is continuous and therefore, phase of water in the signal region can be aligned by phase correction, further, phase of fat can also be aligned. Therefore, in the case of a single signal region, the water image and fat image can properly be formed to separate. However, when a signal region and another signal region are divided by a noise region, a phase error caused by a non-uniformity in a magnetic field in one signal region and a phase error caused by a non-uniformity in a magnetic field in other signal region, become discontinuous and therefore, although the phase of water can be aligned in the respective signal region, the phase of water cannot be aligned among the signal regions, further, although the phase of fat can be aligned in the respective signal region, the phase of fact cannot be aligned among the signal regions. Therefore, when there are a plurality of signal regions, the water image and the fat image cannot be properly formed to separate.
Hence, according to the first aspect of the water and fat separation image forming method, the phase correction is respectively carried out in the respective signal regions, thereafter, the phase shifts among the respective signal regions are corrected and thereafter, the water image and the fat image are separately formed by utilizing the phase difference between water and fat. By correction of the phase shift, the phase of water can be aligned in all of the signal regions, further, the phase of fat can also be aligned. Therefore, even when there are present the plurality of signal regions, the water image and the fat image can properly be formed to separate.
According to a second aspect of the present invention, there is provided the water and fat separation image forming method according to the first aspect, wherein in correcting the phase shifts, reference peak phases are respectively detected with respect to distributions of pixel numbers with regard to the phases of the respective signal regions subjected to the phase correction and the distributions of the pixel numbers with regard to the phases of the respective signal regions are shifted such that the reference peak phases coincide with each other.
Taking a distribution of a pixel number with respect to a phase in a respective signal region subjected to phase correction, the pixel number becomes a peak at a phase of water and a phase of fat.
Therefore, according to a second aspect of the water and fat separation image forming method, for example, as reference peak phases, phases of peaks of water are detected and distributions of the pixel numbers with respect to the phases of the respective signal regions are shifted such that the peak phases of water in the respective signal regions coincide with each other. Thereby, the phase of water can be aligned in all of the signal regions, further, the phase of fat can also be aligned. Therefore, even when there are present the plurality of signal regions, the water image and the fat image can properly be formed to separated.
According to a third aspect of the present invention, there is provided a magnetic resonance imaging apparatus characterized in comprising imaging means for taking a complex number image by utilizing magnetic resonance to provide a phase difference between water and fat, phase correcting means for respectively carrying out phase correction in respective signal regions when there are present a plurality of the independent signal regions in the complex number image, phase shift correcting means for correcting phase shifts among the respective signal regions subjected to the phase correction and image forming means for separately forming a water image and a fat image by utilizing the phase difference between water and fat from the complex number image corrected with the phase shifts.
According to the third aspect of the magnetic resonance imaging apparatus, the water and fat separation image forming method according to the first aspect can preferably be implemented.
According to a fourth aspect of the present invention, there is provided the magnetic resonance imaging apparatus according to the third aspect, characterized in that the phase difference between water and fat is not xcfx80, and wherein the phase shift correcting means comprises phase distribution forming means for forming distributions of pixel numbers with regard to phases at the respective signal regions subjected to phase correction, added distribution forming means for respectively calculating added distributions by shifting the phase distributions inversely by the phase difference and adding the shifted respective phase distributions to original phase distributions, reference peak phase determining means for calculating phases constituting maximum values of the pixel numbers in the respective added distributions and determining the phases as reference peak phases in original phase distributions and phase distribution shifting means for shifting the phase distributions in the respective signal regions such that the respective reference peak phases coincide with each other.
Taking a distribution of a pixel number with respect to the phase in the respective signal region subjected to phase correction, the pixel number is to constitute a peak at a phase of water and a phase of fat. However, in actually measured data, there are present a number of peaks, there emerges a peak larger than a peak in the phase of water or the phase of fat, a top portion of a peak is flat. It is not easy to determine which is the peak of water or which is the peak of fat. That is, it is not easy to calculate phase shifts among the signal regions.
Hence, according to the fourth aspect of the magnetic resonance imaging apparatus, the respective phase distribution subjected to the phase correction is shifted inversely by a phase difference between water and fat and added to the original phase distribution to thereby respectively calculate the added distribution. Thereby, in the respective added distribution, the peak of water and the peak of fat overlap at one location. A probability of constituting a maximum peak by the one overlapped portion is extremely high. Therefore, when a phase constituting a maximum value of the pixel number is calculated, the phase is the phase of water or the phase of fat (either of them may constitute a reference), the reference peak phase in the original phase distribution can easily be calculated. Further, the phase shifts among the respective signal regions can easily be calculated as differences among the reference peak phases of the respective signal regions. When the phase distributions of the respective signal regions are shifted to coincide these, the phase shifts among the respective signal regions can be corrected.
Further, the reason that the phase difference between water and fat is not xcfx80, is that when the phase difference is, the peak of water and the peak of fat cannot be discriminated from each other.
According to a fifth aspect of the present invention, there is provided a reference peak phase detecting method characterized in that when there is a phase distribution including a reference peak constituting a first maximum value of a value Y in a distribution of the value Y with respect to a phase xcex8 and a referred peak constituting a second maximum value of the value Y at a phase at which the phase xcex8 differs from the reference peak by xcex1 (xcex1 is not equal to xcfx80), the phase distribution is shifted inversely by xcex1 and added to an original phase distribution to thereby calculate an added distribution, a phase xcex8 max constituting a maximum value of the Y in the added distribution is calculated and the phase xcex8 max is determined as a phase of the reference peak in the original phase distribution.
The fifth aspect of the reference peak phase detecting method, is a method of simply detecting a phase of a reference peak when there is a phase distribution including a reference peak constituting a first maximum value of a value Y in a distribution of the value Y with respect to a phase xcex8 (the value Y may be any value) and a referred peak constituting a second maximum value of the value Y at a phase at which the phase xcex8 differs from the reference peak by xcex1 (xcex1 is not xcfx80).
That is, in the actually measured data of the distribution of the value Y with respect to the phase xcex8, a number of peaks are present, a peak larger than the reference peak or the referred peak emerges, a top portion of a peak is flat. Accordingly, it is not easy to determine which is the reference peak.
Hence, according to the fifth aspect of the reference peak phase detecting method, the phase distribution is shifted inversely by a phase difference between the reference peak and the referred peak and added to the original phase distribution to thereby calculate the added distribution. Thereby, according to the added distribution, the reference peak and the referred peak overlap at one location. A probability of constituting a maximum peak at the one overlap location is extremely high. Therefore, when a phase constituting the maximum value of the pixel number is calculated in the added distribution, the phase is the phase of the reference peak and the reference peak phase in the original phase distribution can easily be detected.
Further, the reason that the phase difference between the reference peak and the referred peak is not xcfx80, is that when the phase difference is xcfx80, the reference peak and the referred peak cannot be discriminated from each other.
According to a sixth aspect of the present invention, there is provided a reference peak position detecting method characterized in that when there is an XY distribution including a reference peak constituting a first maximum value of a value Y in a distribution of the value Y with respect to a position X and a referred peak constituting a second maximum value of the value Y at a position at which the position X differs from the reference peak by A, the XY distribution is shifted inversely by A and added to an original XY distribution to thereby calculate an added distribution, a position Xmax constituting a maximum value of the value Y in the added distribution is calculated and the position Xmax is determined as a position of the reference peak in the original XY distribution.
The reference peak position detecting method according to the sixth aspect is a method of simply detecting a position of a reference peak when there is a position distribution including a reference peak constituting a first maximum value of a value Y in a distribution of the value Y (the value Y may be any value) with respect to a position X (position X may be any value, for example, may be a phase, may be a frequency or may be a distance) and a referred peak constituting a second maximum value of the value Y at a position at which the position X differs from the reference peak by A.
That is, in the actually measured data of the distribution of the value Y with respect to the position X, a number of peaks are present, a peak larger than the reference peak or the referred peak emerges, a top portion of a peak is flat. Accordingly, it is not easy to determine which is the reference peak.
Hence, according to the sixth aspect of the reference peak position detecting method, the position distribution is shifted inversely by a position difference between the reference peak and there referred peak and added to the original position distribution to thereby calculate the added distribution. Thereby, according to the added distribution, the reference peak and the referred peak overlap at one location. A probability of constituting a maximum peak at the one overlapped location is extremely high. Therefore, when a position of constituting the maximum value of the pixel number is calculated in the added distribution, the position is the position of the reference peak and the reference peak position in the original position distribution can easily be detected.
According to the water and fat separation image forming method and the magnetic resonance imaging apparatus of the invention, even in the case in which a plurality of independent signal regions are present in a complex number image taken by utilizing magnetic resonance, a water image and a fat image can be formed separately and preferably.
According to the reference peak phase detecting method and the reference peak position detecting method of the invention, a phase or a position of a reference peak in a distribution satisfying a specific condition can easily be detected.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.